1. Field of the Invention
This invention is directed to a liquid solder flux, and, more particularly, to an organic acid activated liquid solder flux particularly useful in the electrical and electronics industries.
2. Description of the Prior Art
Soldering with a soft solder, i.e., a tin-lead based alloy melting at below 400.degree. C., is widely employed in the electrical and electronics industries, for example, in the assembly of printed circuits, electronic components and conductors. In order to produce a satisfactory soldered joint, it is necessary to use a flux with the soft solder in order to remove any residual surface oxide films and hence provide a clean surface and in order to reduce the surface tension of the molten solder and hence promote good wetting of the surface by the solder. Fluxes for soft soldering are classified as corrosive, intermediate, or non-corrosive. It is normally essential in the electronics industry to use a flux which is non-corrosive, i.e., a flux which after use in a soldering operation yields a residue which is substantially inert and hence will not appreciably corrode the soldered joint, particularly under humid conditions. Non-corrosive fluxes are conventionally natural rosin-based fluxes comprising wood rosin and gum (or water-white gum) rosin. Rosin principally comprises a mixture of rosin acids, the major component of which is an abietic acid. The rosin flux may contain a small amount of an additive, generally known as an activating agent, which will improve the fluxing action of the rosin. Such rosin fluxes may be incorporated as a core or cores in solder wires, or may be used in the form of solutions or pastes.
The ingredients of an activated rosin increase the reaction to metal oxides and make wetting by the solder easier and faster. Ideally, any residue of the activating agent boils off with the heat of soldering, and what remains is solidified rosin, which is non-corrosive and non-conducting.
In certain critical electronic assemblies, use of commercial fluxes has resulted in hardware damage (e.g., lifted pads on printed wiring boards and corroded connectors). Work continues in an effort to develop suitable fluxes that permit making a solder joint faster and better with less hardware damage, consistent with minimal corrosivity of the hardware.